The present invention relates to corrosion inhibitors and more particularly to combinations of substances for use as vapor-phase corrosion inhibitors (volatile corrosion inhibitors, VCI) for protecting conventional metals for use, such as iron, chromium, nickel, tin, zinc, aluminum, copper and alloys thereof, from atmospheric corrosion.
It is already known in general that corrosion inhibitors which tend to undergo sublimation in powder form under normal conditions and can reach metal surfaces that are to be protected through the gas phase, can be used for temporary corrosion prevention on metal objects within closed spaces, e.g., in packaging or in display boxes.
These vapor-phase inhibitors (VPI) or volatile corrosion inhibitors (VCI) are usually selected according to the type of metal to be protected and are used in the form of a powder packaged in a bag of a material that is permeable for the vapor-phase inhibitors (see, for example, H. H. Uhlig, Corrosion and Corrosion Prevention, Akademie-Verlag Berlin, 1970, pages 247-249; K. Barton, Protection Against Atmospheric Corrosion; Theory and Practice, Verlag Chemie, Weinheim 1973, pages 96 ff. or I. L. Rozenfeld, Corrosion Inhibitors (Russian) Izt-vo Chimija, Moscow 1977, page 320 ff; A. D. Mercer, Proceedings of the 7th European Symposium on Corrosion Inhibitors, Ann. Univ. Ferrara/Italy, N. S., Sez V, Suppl. No. 9 (1990), 449 pp.).
Modern packaging materials for corrosion prevention contain VCIs either in tablet form within porous foam capsules or as a fine powder inside of polymer carrier materials. For example, U.S. Pat. Nos. 3,836,077, 3,967,926, 5,332,525, 5,393,457, 4,124,549, 4,290,912, 5,209,869, Japanese Patent 4,124,549, European Patent 0,639,657 and Unexamined German Patent 3,545,473 propose several variants whereby VCIs are introduced in the form of capsules or air-permeable plastic films, either by incorporation into cavities created by cutting open a foam and subsequently covering same with a gas-permeable material or by adding the VCI to the polymer melt intended for melt extrusion or blow molding, thus resulting in a packaging material (film or hard material) out of which the VCI components are able to sublime continuously because of the structurally induced porosity.
There have already been attempts to incorporate VCIs during foaming of polymeric solids, as described for example in Japanese Patent 58,063,732, U.S. Pat. No. 4,275,835 and German Democratic Republic Patent 295,668. In addition, packaging materials containing VCI can be produced by dissolving the VCI components in a suitable solvent and applying this solution to the respective packaging material. Methods of this type using various active ingredients and solvents are described, for example, in Japanese Patents 61,227,188, 62,063,686, 63,028,888, 63,183,182, 63,210,285, German Patent 1521900 and U.S. Pat. No. 3,887,481.
However, the VCI packaging materials produced in this way usually contain the active ingredients incorporated only loosely in the structurally induced cavities in the carrier material, whether paper, cardboard, foam, etc., so there is the danger of mechanical rupturing and escape of the active ingredient particles, so it is impossible to ensure that carrier materials pretreated in this way will still have the required specific surface concentration of VCI at the time of their use for corrosion prevention.
To eliminate this disadvantage, U.S. Pat. No. 5,958,115 describes a corrosion-inhibiting composite material which consists of a mixture of metal oxide sol, corrosion inhibitors that are capable of sublimation and additional additives and forms a firmly adhering, sufficiently porous gel film of the metal oxides and additives used on the support material, so that the corrosion inhibitors (VCIs) are released from the film at a uniform, long-lasting emission rate.
According to the ISO definition, a corrosion inhibitor is a xe2x80x9cchemical substance which decreases the corrosion rate when present in the corrosion system at a suitable concentration without significantly changing the concentration of any other corrosive agent; the use of the term inhibitor should be qualified by the nature of the metal and the environment in which it is effectivexe2x80x9d (cf. Corrosion of metals and alloysxe2x80x94Terms and definitions, ISO 8044-1986).
The main principle in the use of VCIs is to maintain or reinforce the inherent primary oxide layer, which usually provides only limited protection but which forms very rapidly on any metal due to contact with the atmosphere, although it cannot be perceived visually without optical aids (K. Barton, loc. cit.; E. Kunze (eds.), Corrosion and Corrosion Protection, volume 3, Wiley-VCH, Berlin, Weinheim, New York 2001, pages 1680 ff.).
With regard to the type and properties of said primary oxide layer, the known utilitarian metals and their alloys may be divided into two categories, namely the passivatable metals, where a sufficiently strong oxidizing agent is required to maintain or recreate the protective primary oxide layer, and those metals which are classified as non-passivatable, where the passive oxide layer undergoes chemical and/or structural changes due to the action of strong oxidizing agents so that adhesion to the substrate and thus also the corrosion-preventing effect are lost.
To illustrate this distinction between the two categories of utilitarian metals, the following examples shall be used. In the ferrous materials which belong to the category of passivatable metals, the primary oxide layer consists mainly of Fe(III) oxides, for example. If the metal surface becomes moistened, which is the case when a condensed film of water develops in rooms saturated with water vapor due to a drop in temperature when a sufficiently strong oxidizing agent is not in effect at the same time, then corrosion of the metal begins by conversion of these oxides into Fe(II) compounds, e.g.:
Fe2O3+H2O+2H++2exe2x88x922Fe(OH)2
and for the anodic step of corrosion of the substrate metal:
Fe+2H2Oxe2x86x92Fe(OH)2+2H++2exe2x88x92
they function cathodically.
Metals that must be classified in the category of non-passivatable metals include, for example, copper whose primary oxide layer is sensitive to further oxidation. Its primary oxide layer is known to consist mainly of the oxide Cu2O and it is stable only in aqueous media which do not contain any strong oxidizing agent, regardless of pH. Under the action of oxygen in humid air, however, the oxide CuO is formed relatively rapidly and is detectable as a black deposit which cannot become intergrown with the metal substrate because of its crystal lattice dimensions (no epitaxy) and therefore cannot provide any corrosion protection. The following equation can be formulated for the starting reactions of atmospheric corrosion of copper:
Cu2O+H2Oxe2x86x922CuO+2H++2exe2x88x92
xc2xdO2+2H++2exe2x88x92xe2x86x92H2O
and as the gross reaction which eliminates the passive state:
Cu2O+xc2xdO2xe2x86x922CuO
Most conventional utilitarian metals are considered to be passivatable on contact with aqueous media. Thus, the case with nickel is similar to that with iron because its primary oxide layer contains Ni2O3. In the case of chromium, the passive state is caused by Cr2O3/CrOOH, and in the case of tin it is caused by SnO/SnO2, in the case of zinc it is caused by ZnO and in the case of aluminum by Al2O3/AlOOH. These passive oxide layers are usually maintained in neutral aqueous media or they form again spontaneously after local mechanical abrasion (abrasion, erosion) when the action of a sufficiently strong oxidizing agent is guaranteed (E. Kunze, loc. cit.).
Nitrites as salts of nitrous acid have already proven very successful as passivating oxidizing agents of this type. Therefore, they have long been used as vapor-phase inhibitors. The relatively volatile dicyclohexylammonium nitrite has already been in use as a vapor-phase inhibitor for more than 50 years (see Uhlig, Barton, Rozenfeld, Kunze, loc. cit.) and is mentioned as a component of VCI compositions in numerous patent publications (e.g., U.S. Pat. Nos. 2,419,327, 2,432,839, 2,432,840, 4,290,912, 4,973,448, Japanese Patents 02085380, 62109987, 63210285 A and German Patent 4040586). The effect of the nitrite ion as an oxidizing agent is associated with its electrochemical reduction, for which the following reactions may be formulated, for example:
2NO2xe2x88x92+2H++2exe2x88x922NO+2OHxcx9c
NO2xe2x88x92+3H2O+2H++6excx9cNH3+5OHxcx9c
Since these reactions lead only to the formation of hydroxyl ions, OHxcx9c, they proceed less intensely in aqueous media the higher the prevailing pH of the medium.
From this standpoint it is not advantageous that dicyclohexylamine or the dicyclohexylammonium ion formed by dissociation of dicyclohexylammonium nitrite establishes pH values of approx. 9 in water at room temperature. This is not only a disadvantage for the manifestation of the passivator effect of the nitrite but also endangers the stability of the passive oxide layer of zinc and aluminum materials. The oxides of these metals are known to be stable only in a neutral pH range, and they undergo progressive dissolution at a pH greater than 8, forming zincate or aluminate:
xe2x80x83ZnO+H2O+OHxcx9cZn(OH)3xcx9c
Al2O3+H2O+5OHxcx9c2Al(OH)4xcx9c
In the attempt to create VCI packaging materials which can be used not only for iron metals but also at least for galvanized steels and aluminum materials, there have been attempts to formulate VCI combinations which contain not only amine nitrites but also components which have a pH regulating effect in condensed water films on metal surfaces, so the dissolution of the passive oxide layers described above cannot occur.
From this standpoint, it has been proposed that nitrite-amine mixtures should be combined with other substances that are capable of sublimation, such as the salts of weak to medium-strong, saturated or unsaturated carboxylic acids, as described, for example, in U.S. Pat. Nos. 2,419,327, 2,432,839, 2,432,840 and German Patent 814,725. To be sure, this yields improved protection of the usual Al- and Zn-materials when they are in contact with an aqueous medium or film of condensed water if the passive oxide layer is not damaged mechanically or dissolved by action of chelating agents, but the passivating properties of the nitrite are also reduced by this species at the same time. The respective carboxylates are known to create pH buffering systems of a higher buffering capacity in aqueous media or films of condensed water on metal surfaces, with or without the simultaneous presence of an amine in the absence of the respective carboxylic acid/salt system, and thus they prevent the reducibility of oxidizing agents, which is evident in principle from the reduction reactions for nitrite given above. These reactions, which are necessary for the passivation effect, are known to proceed from left to right voluntarily only if the respective reaction medium does not already have a high concentration of OHxe2x88x92 ions or if the OHxe2x88x92 ions that are formed are regularly removed from the medium, or if the concentration of the oxidizing agent in the medium remains comparatively much higher than that of the OHxe2x88x92 ions formed, e.g., by virtue of the fact that the amount of oxidizing agent converted is continuously re-supplied from a source.
All the traditional applications of VCI combinations which also contain an amine or amine carboxylate in addition to an oxidizing agent such as nitrite, chromate or an organic nitro compound, may consequently be successful in practical implementation only if the oxidizing agent which has a passivating effect is used in excessive concentrations. However, this fact is not always readily apparent from the corresponding patent literature, because the concentration ranges in which the VCI combinations according to this invention may be used are generally stated very generously. Such VCI combinations containing oxidizing agents are described, for example, in GB Patent 600,328, where it is recommended that as much organic nitrite salt as possible should be used, or in German Patent 814725, where nitrite salts of organic nitrogenous bases (e.g., carboxylates, piperidines, oxazines or morpholines) are proposed under the condition that at least 0.5 to 20 g of the nitrite should be applied per square meter of packaging material, and reliable protection is achieved only when at least 35 to 600 grams of this substance are emitted per cubic meter of the interior of the package.
Practical use of the oxidizing agents mentioned above is regulated today due to their known, relatively harmful effects on people and the environment, so there are limits with regard to the concentration in preparations and the maximum allowed job site concentration (MAK value) (e.g., classification of substances and preparations according to EC Guideline 67/548/EEC including annual updates). Therefore, the VCI combinations mentioned here with excessive passivator amounts can no longer be used.
As a replacement for this, U.S. Pat. Nos. 5,209,869 and 5,332,525 and European Patent 0662527 A1 have already proposed that the VCI mixtures consisting of nitrites and amine carboxylates, with or without molybdate, should also be combined with a desiccant such as silica gel, so that the development of a condensed film of water on the metal surface to be protected and the related negative pH effect can be postponed for the longest possible amount of time. However, this proposal has the significant disadvantage that the VCI system fixed in or on the packaging material has a great tendency to absorb water from the environment due to the presence of the desiccant, which in turn leads to a negative effect on the emission rate of VCI components and thus to a reduction in the VCI corrosion-preventing effect.
On the other hand, with the increasing globalization and intertwining of the economic regions throughout the world, the demand for reliably functioning VCI systems and VCI packaging materials has greatly increased, and the use of VCI in storage and shipping processes has become much more environmentally friendly and inexpensive than the methods of temporary corrosion protection known in the past, which consisted of applying oils, fats and waxes, and whereby at the time of removal of these agents from the metal parts, large quantities of organic solutions that were difficult to dispose of were obtained.
Most of the VCI systems known in the past, which contain a nitrite and an amine at the same time, cannot yield the required reliability for the reasons mentioned above. Another uncertainty factor that has developed in the meantime is that especially the secondary amines and cyclic nitrogenous compounds such as morpholine and piperidine, which have been introduced as VCI components, are readily converted to N-nitroso compounds. These N-nitrosamines usually react as weak oxidizing agents and promote corrosion of metals. However, their carcinogenic effect is a much more important disadvantage which prevents large-scale industrial use of these VCI systems.
At first an attempt was made to eliminate this disadvantage by replacing the nitrite, because it was assumed that nitrosation of amines is caused only by the simultaneous presence of nitrite. U.S. Pat. No. 4,051,066 therefore proposes the use of m-nitrobenzoate and dinitrobenzoate instead of nitrite, while German Democratic Republic Patents 268978 and 295668 propose the use of dicyclohexylamine-o-nitrophenolate and dicyclohexylamine-m-nitrobenzoate. Finally, GB Patent No. 1,224,500 generalizes regarding the use of volatile aliphatic and aromatic nitro compounds together with heterocyclic amines and mentions 2-nitropropane, nitrobenzene and dinitrobenzene specifically. First, however, the passivator properties of these alternative oxidizing agents have proven to be much weaker in comparison with those of nitrite and secondly, the intended effect of avoiding the formation of N-nitrosamine with the amines used at the same time was not achieved. In the meantime, it is known that such well-proven VCI components as morpholine and dicyclohexylamine undergo nitrosation due to the normal constituents of air, in particular in contact with metals and at high temperatures. This virtually prevents their incorporation into plastics, because melt extrusion, injection molding or blow molding are known to be performed at temperatures around 200xc2x0 C. in metallic installations.
To satisfy the demand for films and hard plastics finished with VCI for handling overseas shipments, it has been proposed that amine-free VCI systems containing nitrite be used. For example, U.S. Pat. No. 3,836,077 describes a combination of nitrite with borate and a phenol which is mono-, di- or trisubstituted with styrene. The purpose of using such phenols with aromatic substituents was not explained in this patent specification, but it can be assumed that they are intended to function as antioxidants merely to ensure the stability of the polyolefin films against the oxidative effect of the nitrite which is present in large amounts. Only small amounts of nitrite will sublime out of films produced from polyethylene and combinations thereof as long as the phenyl-beta-naphthylamine, which is also claimed in that patent specification, is not additionally incorporated. The emission rate of the nitrite is improved by the presence of this amine, but this does not meet the goal of remaining amine-free. Furthermore, with this amine it is not possible to achieve sublimation of borate and the aromatically substituted phenols.
U.S. Pat. No. 4,290,912, however, emphasizes the use of inorganic nitrites in combination with a triple-substituted phenol and silica gel for production of VCI films, but the embodiments prove that in the case of phenols, only aliphatically substituted phenols and especially 2,6-di-tert-butyl-4-methylphenol (butylated hydroxytoluene, BHT) are intended. Since these substituted phenols have a tendency to sublimation even at normal temperature, an improved sublimation rate can be achieved with this combination, even for sodium nitrite or potassium nitrite, without the involvement of a volatile amine, but the nitrite reaching the metal surface cannot achieve reliable VCI corrosion protection without the use of additional components. In the case of passivating metals, it is necessary to have the cooperation of components which adjust the pH in condensed water films in a range that is favorable for passivation and which stabilize the passive oxide layer that is formed by adsorption to prevent dissolution (see, for example, E. Kunze, loc. cit.). In the simultaneous presence of non-passivating metals such as copper materials, exclusive action of a nitrite would also result in increased corrosion.
Benzotriazole has long been used for protecting copper and copper alloys from atmospheric corrosion (see, for example, Barton, Mercer, loc. cit.). However, since the sublimation tendency of this compound is relatively low, German Patent 1182503 and U.S. Pat. No. 3,295,917 propose that the source of this VCI should first be adjusted to a higher temperature (up to approx. 85EC.) and at the same time the metal objects on which condensation is to take place should be cooled. U.S. Pat. Nos. 2,941,953 and 3,887,481, however, describe the impregnation of paper with benzotriazole and/or tolyltriazole. Organic solvents such as tetrachloroethylene are used, and it is specified that the metal parts to be protected should be wrapped as tightly and as closely as possible with the VCI packaging material impregnated in this way to minimize the distance between the VCI source and the metal surface to be protected. However, this technology has the disadvantage mentioned above that the active ingredient in the form of extremely fine particles of powder does not adhere well to the paper and can easily slip off, so the corrosion-preventing properties of this packaging material cannot be reliable.
The sublimation tendency of benzotriazole and tolyltriazole from VCI source also increases, like that of inorganic nitrites and nitrates, when other sublimable solids in powder form are also incorporated at the same time. In this regard, European Patent 0662527 mentions mixtures of benzotriazole with cyclohexylaminebenzoate and ethylaminebenzoate or with anhydrous sodium molybdate and dicyclohexylamine nitrite, while U.S. Pat. No. 4,051,066 and U.S. Pat. No. 4,275,835 mention mixtures of benzotriazole with ammonium molybdate and amine molybdates, aminebenzoates and nitrates, U.S. Pat. No. 4,973,448 describes mixtures of benzotriazole with organic carbonates, phosphates and amines; finally, Japanese Patents 62063686 and 63210285 A mention mixtures of benzotrizaole with alkali and amine salts of aromatic carboxylic acids.
Combinations of benzotriazole, tolyltriazole or methyl-benzotriazole with other volatile organic nitrogen solids are described, for example, in Japanese Patents 62109987 and 61015988, German Democratic Republic Patents 268978 and 298662. One disadvantage is that all the components containing amine and ammonium ions reduce the protective effect of triazoles, especially with regard to the nonferrous metals because of their rather pronounced tendency to form complexes with metal ions. In addition, these amines and ammonium compounds are highly hydrophilic. VCI sources containing such substances have a tendency to increased uptake of water, as already mentioned above. Their hydrolysis then usually results in a marked reduction in their sublimation tendency, which necessarily results in a reduction in the corrosion-preventing effect.
To utilize the advantages of using VCI and the inhibitor effect of the triazole structure, Japanese Patent 03079781 proposes that instead of the substance combinations of triazole and amine, only alkylaminotriazoles should be used. In fact, the substances mentioned explicitly, namely 3-amino-1,2,4-triazole and 3-amino-5-methyl-1,2,4-triazole, have a higher rate of volatilization, but do not have such a definite corrosion-preventing effect with respect to copper as do benzotriazole and tolyltriazole.
Further vapor-phase corrosian inhibitors are described in DE 39 40 803, DE 199 03 400, DE 100 13 471, U.S. Pat. No. 4,200,542, EP 522 161 and JP 05-093 286.
If hard plastics and plastic films equipped with VCI components are to be made available for modern packaging, shipping and storage technologies, and if VCI additives which are capable of guaranteeing VCI corrosion protection for the broadest possible range of utilitarian metals are to be used, then essentially the following problems must be overcome for their production:
first, the high volatility of the VPI at temperatures at which the extrusion process is performed must be calculated into the process, because this can lead to extensive transfer of the inhibitors to the gaseous state and thus to significant losses of these substances and to foaming of the film, as well as violation of its intactness and thus to uncontrolled reduction in its strength and protective properties;
secondly, it should be recalled that thermal decomposition of the corrosion inhibitors and chemical reactions of the components with one another and with the polymer matrix may occur in the course of processing of these mixtures during the extrusion process. This results on the whole in the significant advantage that many of the VPIs customary in the past are no longer applicable in this way and must be replaced by new types of active ingredients.
The object of this invention is to provide sublimable corrosion-inhibiting substances and substance combinations that are improved in comparison with the traditional corrosion inhibitors whose advantages are described above, such that the substances and combinations of substances will sublime from the corresponding source in particular under climate conditions that are of practical interest inside industrial packages and similar closed spaces at an adequate rate, and after adsorption and/or condensation on the surface of metals in said space, said substances will ensure conditions therein under which the conventional utilitarian metals will be reliably protected from atmospheric corrosion. Furthermore, another object of this invention is to provide methods of producing and processing such substances and substance combinations for production of improved VCI packaging materials.
These and other objects are achieved in accordance with the present invention which comprises a corrosion-inhibiting substance combination containing an inorganic salt nitric acid; a water-insoluble polysubstituted phenol; an aliphatic ester of a dihydroxybenzylic acid; and tocopherol (2,5,7,8-tetramethyl-2-(4xe2x80x2,8xe2x80x2,12xe2x80x2-trimethyltridecyl) chroman-6-ol). The present invention also comprises a method of producing a corrosion-inhibiting substance combination that is capable of sublimation, wherein an inorganic salt of nitric acid; a water-insoluble polysubstituted phenol; an aliphatic ester of a dihydroxybenzylic acid; and tocopherol are mixed together.